Lubricating oil compositons

ABSTRACT

Disclosed is a natural gas engine lubricating oil composition which comprises: (a) a major amount of an oil of lubricating viscosity having a kinematic, (b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants, (c) about 0.1 to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants, (d) one or more metal dithiophosphates, and (e) one or more alkali metal or alkali earth metal phenate detergents having a TBN of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/690,577, filed Jun. 27, 2018.

TECHNICAL FIELD

The disclosed technology relates to lubricants for a natural gas engine with steel pistons.

BACKGROUND OF THE DISCLOSURE

Natural gas fueled engines are engines that use natural gas as a fuel source. Lubricating oils with high resistance to oxidation, nitration and viscosity increase are generally preferred for lubricating oils used in natural gas engines because of the conditions related to this type of engine.

Natural gas has a higher specific heat content than liquid hydrocarbon fuels and therefore it will burn hotter than liquid hydrocarbon fuels under typical conditions. In addition, since it is already a gas, natural gas does not cool the intake air by evaporation as compared to liquid hydrocarbon fuel droplets. Furthermore, many natural gas fueled engines are run either at or near stoichiometric conditions, where less excess air is available to dilute and cool combustion gases. As a result, natural gas fueled engines generate higher combustion gas temperatures than engines burning liquid hydrocarbon fuels. In most cases, stationary natural gas fueled engines are used continuously at 70 to 100% load, whereas an engine operating in vehicular service may only spend 25% of its time at full load.

Recently natural gas engine manufacturers are developing engines that have greater power density or higher power produced per unit of displacement. This has led to increased pressure in the engine cylinders. Original engine manufacturers (OEMs) have also moved the first ring of the piston closer to the piston top to reduce the dead space down the crownland of the piston and to reduce the emissions generated by the engine. The increased severity with regards to both pressure and temperature has led OEMs to replace aluminum pistons with steel pistons. Steel is being used to increase the strength of the pistons which are subjected to high pressures and temperatures and must be able to withstand limited detonation (i.e., uncontrolled fuel ignition/shock waves collisions). Lubricants for use in natural gas engines lubricate the movement of the engine, including the movement of the pistons within the cylinders. These lubricants are exposed to extremely high temperatures in upper portion of the pistons due to the proximity to the combustion area. The combustion area on the piston top can range in temperatures from about 1200° C. to 2000° C., depending on, for example, the British Thermal Unit (BTU) quantity of the natural gas, lean or rich burn strategy, and load. As the manufacturers move to engines with increased output or power density, the lubricant is exposed to increasingly severe conditions. In steel piston engines with Brake Mean Effective Pressure (BMEP) greater than 20 bar, additional deposits and shorter lubricant life cycles have been observed.

Accordingly, despite the advances in lubricant oil formulation technology, there exists a need to prevent or inhibit the formation of deposits in a natural gas engine with steel pistons.

SUMMARY OF THE DISCLOSURE

In accordance with one illustrative embodiment, a natural gas engine lubricating oil composition is provided which comprises:

(a) a major amount of an oil of lubricating viscosity,

(b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants,

(c) about 0.1 wt. % to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants,

(d) one or more metal dithiophosphates, and

(e) one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

In accordance with another illustrative embodiment, a method for preventing or inhibiting deposit formation in a natural gas engine containing one or more steel pistons is provided comprising the step of operating the natural gas engine with a natural gas engine lubricating oil composition comprising:

(a) a major amount of an oil of lubricating viscosity,

(b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants,

(c) about 0.1 wt. % to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants,

(d) one or more metal dithiophosphates, and

(e) one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

In accordance with another illustrative embodiment, there is provided a use of a natural gas engine lubricating oil composition for preventing or inhibiting deposit formation in a natural gas engine containing one or more steel pistons, wherein the natural gas engine lubricating oil composition comprises:

(a) a major amount of an oil of lubricating viscosity,

(b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants,

(c) about 0.1 to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants,

(d) one or more metal dithiophosphates, and

(e) one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

The natural gas engine lubricating oil compositions of the present disclosure advantageously prevents or inhibits deposit formation in a natural gas engine containing one or more steel pistons.

DETAILED DESCRIPTION OF THE DISCLOSURE

To facilitate the understanding of the subject matter disclosed herein, a number of terms, abbreviations or other shorthand as used herein are defined below. Any term, abbreviation or shorthand not defined is understood to have the ordinary meaning used by a skilled artisan contemporaneous with the submission of this application.

Definitions

In this specification, the following words and expressions, if and when used, have the meanings given below.

A “major amount” means in excess of 50 wt. % of a composition.

“Active ingredients” or “actives” refer to additive material that is not diluent or solvent.

All percentages reported are weight % on an active ingredient basis (i.e., without regard to carrier or diluent oil) unless otherwise stated.

The term “ppm” means parts per million by weight, based on the total weight of the lubricating oil composition.

Kinematic viscosity at 100° C. (KV₁₀₀) was determined in accordance with ASTM D445.

The term “metal” refers to alkali metals, alkaline earth metals, or mixtures thereof.

The term “alkali metal” refers to lithium, sodium, potassium, rubidium, and cesium.

Overbased detergents may be low overbased (LOB), e.g., an overbased salt having a TBN below 100 mgKOH/g on an actives basis. The TBN of LOB detergents may be from about 30 to about 100 mgKOH/g.

Overbased detergents may be medium overbased (MOB). The TBN of MOB detergents may be from about 100 to about 200 mgKOH/g on an actives basis.

Overbased detergents may be high overbased (HOB). The TBN of HOB detergents may be from about 250 to about 800 mgKOH/g on an actives basis.

The term “alkaline earth metal” refers to calcium, barium, magnesium, and strontium.

Throughout the specification and claims the expression oil soluble or dispersible is used. By oil soluble or dispersible is meant that an amount needed to provide the desired level of activity or performance can be incorporated by being dissolved, dispersed or suspended in an oil of lubricating viscosity. Usually, this means that at least about 0.001% by weight of the material can be incorporated in a lubricating oil composition. For a further discussion of the terms oil soluble and dispersible, particularly “stably dispersible”, see U.S. Pat. No. 4,320,019 which is incorporated herein by reference.

The term “Total Base Number” or “TBN” as used herein refers to the amount of base equivalent to milligrams of KOH in one gram of sample. Thus, higher TBN numbers reflect more alkaline products, and therefore a greater alkalinity. TBN was determined using ASTM D 2896 test, and reported on an oil-free basis.

Calcium, phosphorus, and sulfur contents were determined in accordance with ASTM D5185.

The term “Normal Alpha Olefins” refers to olefins which are straight chain, non-branched hydrocarbons with carbon-carbon double bond present in the alpha or primary position of the hydrocarbon chain.

The term “Isomerized Normal Alpha Olefin” refers to an alpha olefin that has been subjected to isomerization conditions which results in an alteration of the distribution of the olefin species present and/or the introduction of branching along the alkyl chain. The isomerized olefin product may be obtained by isomerizing a linear alpha olefin containing from, for example, about 10 to about 40 carbon atoms, or from about 20 to about 28 carbon atoms, or from about 20 to about 24 carbon atoms.

The present disclosure is directed to a natural gas engine lubricating oil composition for inhibiting or preventing deposit formation in a natural gas engine containing one or more steel pistons. The natural gas engine may be a two-stroke engine, a three-stroke engine, a four-stroke engine, a five-stroke engine, or a six-stroke engine. The engine may also include any number of combustion chambers, steel pistons, and associated cylinders (e.g., 1 to about 24). For example, in certain embodiments, the engine may be a large-scale industrial reciprocating engine having 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 16, 18, 20, or 24 or more steel pistons reciprocating in cylinders. In certain embodiments, the steel piston may be any steel piston such as, for example, steel or any of a variety of steel alloys, such as 42CrMo4V or 38MnVS6.

The natural gas engines to which the present disclosure is applicable may be characterized as those operated on, i.e., fueled by, natural gas and include internal combustion engines. The natural gas engine may be a stationary natural gas engine, a stationary biogas engine, a stationary landfill gas engine, a stationary unconventional natural gas engine, or a dual-fuel engine. In one embodiment, the internal combustion engine is a stationary engine used in, for example, well-head gas gathering, compression, and other gas pipeline services; electrical power generation (including co-generation); and irrigation.

The natural gas engine lubricating oil composition of the present disclosure may be utilized in preventing or inhibiting deposits in natural gas engines operating under high sustained load conditions such as, for example, a Brake Mean Effective Pressure (BMEP) of at least about 20 bar (2.0 MPa), or about at least 22 bar (2.2 MPa), or at least about 24 bar (2.4 MPa), or at least about 26 bar (2.6 MPa). In one embodiment, the natural gas engine lubricating oil composition of the present disclosure may be utilized in preventing or inhibiting deposits in natural gas engines operating under a BMEP of, for example, about 20 to about 30 bar (about 2.0 to about 3.0 MPa), or about 22 to about 30 bar (about 2.2 to about 3.0 MPa), or about 22 to about 28 bar (about 2.2 to about 2.8 MPa), or about 24 to about 30 bar (about 2.4 to about 3.0 MPa).

The natural gas engine lubricating oil composition of the present disclosure may provide advantaged deposit control performance in any of a number of mechanical components of an engine in addition to the one or more steel pistons. For example, the mechanical components may be a piston ring, a cylinder liner, a cylinder, a cam, a tappet, a lifter, a gear, a valve, a valve guide, or a bearing including a journal, a roller, a tapered, a needle, or a ball bearing. In some aspects, the mechanical component comprises steel.

The natural gas engine lubricating oil composition in accordance with the present disclosure can have a TBN of about 10 or less. In one embodiment, the natural gas engine lubricating oil composition in accordance with the present disclosure can have a TBN of about 5 to about 8. In one embodiment, the natural gas engine lubricating oil composition in accordance with the present disclosure can have a TBN of about 7 to about 8.

In general, the level of sulfur in the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 0.7 wt. %, based on the total weight of the lubricating oil composition, e.g., a level of sulfur of about 0.01 wt. % to about 0.70 wt. %, or about 0.01 wt. % to about 0.6 wt. %, or about 0.01 wt. % to about 0.5 wt. %, or about 0.01 wt. % to about 0.4 wt. %, or about 0.01 wt. % to about 0.3 wt. %, or about 0.01 wt. % to about 0.2 wt. %, or about 0.01 wt. % to about 0.10 wt. %, based on the total weight of the lubricating oil composition. In one embodiment, the level of sulfur in the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 0.60 wt. %, or less than or equal to about 0.50 wt. %, or less than or equal to about 0.40 wt. %, or less than or equal to about 0.30 wt. %, or less than or equal to about 0.20 wt. %, or less than or equal to about 0.10 wt. %, based on the total weight of the lubricating oil composition.

In one embodiment, the level of phosphorus in the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 0.3 wt. %, based on the total weight of the lubricating oil composition. In one embodiment, the level of phosphorus in the natural gas engine lubricating oil compositions of the present disclosure is from about 0.01 wt. % to about 0.3 wt. %, based on the total weight of the lubricating oil composition. In one embodiment, the level of phosphorus in the natural gas engine lubricating oil compositions of the present disclosure is from about 0.01 wt. % to about 0.1 wt. %, based on the total weight of the lubricating oil composition. In one embodiment, the level of phosphorus in the natural gas engine lubricating oil compositions of the present disclosure is from about 0.015 wt. % to about 0.05 wt. %, based on the total weight of the lubricating oil composition.

In one embodiment, the level of sulfated ash produced by the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 1 wt. % as determined by ASTM D 874, e.g., a level of sulfated ash of from about 0.10 wt. % to about 1 wt. % as determined by ASTM D 874. In one embodiment, the level of sulfated ash produced by the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 0.9 wt. % as determined by ASTM D 874, e.g., a level of sulfated ash of from about 0.10 wt. % to about 0.9 wt. % as determined by ASTM D 874. In one embodiment, the level of sulfated ash produced by the natural gas engine lubricating oil compositions of the present disclosure is less than or equal to about 0.8 wt. % as determined by ASTM D 874, e.g., a level of sulfated ash of from about 0.10 wt. % to about 0.8 wt. % as determined by ASTM D 874, or from about 0.65 wt. % to about 0.8 wt. % as determined by ASTM D 874.

In general, the natural gas engine lubricating oil composition of the present disclosure contains at least (a) a major amount of an oil of lubricating viscosity, (b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants, (c) about 0.1 wt. % to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants, (d) one or more metal dithiophosphates, and (e) one or more alkali metal or alkali earth metal phenate detergents having a TBN of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

The natural gas engine lubricating oil composition in accordance with the present disclosure includes an oil of lubricating viscosity (sometimes referred to as “base stock” or “base oil”). The expression “base oil” as used herein shall be understood to mean a base stock or blend of base stocks which is a lubricant component that is produced by a single manufacturer to the same specifications (independent of feed source or manufacturer's location); that meets the same manufacturer's specification; and that is identified by a unique formula, product identification number, or both. The oil of lubricating viscosity is the primary liquid constituent of a lubricant, into which additives and possibly other oils are blended, for example to produce a final lubricant (or lubricant composition). A base oil is useful for making concentrates as well as for making lubricating oil compositions therefrom, and may be selected from natural and synthetic lubricating oils and combinations thereof.

Natural oils include animal and vegetable oils, liquid petroleum oils and hydrorefined, solvent-treated mineral lubricating oils of the paraffinic, naphthenic and mixed paraffinic-naphthenic types. Oils of lubricating viscosity derived from coal or shale are also useful base oils.

Synthetic lubricating oils include hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-hexenes), poly(1-octenes), and poly(1-decenes)); alkylbenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, and di(2-ethylhexyl)benzenes); alkylated naphthalene; polyphenols (e.g., biphenyls, terphenyls, and alkylated polyphenols); and alkylated diphenyl ethers and alkylated diphenyl sulfides and the derivatives, analogues and homologues thereof.

Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids (e.g., malonic acid, alkyl malonic acids, alkenyl malonic acids, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, fumaric acid, azelaic acid, suberic acid, sebacic acid, adipic acid, linoleic acid dimer, and phthalic acid) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, and propylene glycol). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid.

Esters useful as synthetic oils also include those made from C₅ to C₁₂ monocarboxylic acids and polyols, and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol and tripentaerythritol.

The base oil may be derived from Fischer-Tropsch synthesized hydrocarbons. Fischer-Tropsch synthesized hydrocarbons are made from synthesis gas containing H₂ and CO using a Fischer-Tropsch catalyst. Such hydrocarbons typically require further processing in order to be useful as the base oil. For example, the hydrocarbons may be hydroisomerized; hydrocracked and hydroisomerized; dewaxed; or hydroisomerized and dewaxed; using processes known to those skilled in the art.

Unrefined, refined and re-refined oils can be used in the present lubricating oil composition. Unrefined oils are those obtained directly from a natural or synthetic source without further purification treatment. For example, a shale oil obtained directly from retorting operations, a petroleum oil obtained directly from distillation or ester oil obtained directly from an esterification process and used without further treatment would be unrefined oil. Refined oils are similar to the unrefined oils except they have been further treated in one or more purification steps to improve one or more properties. Many such purification techniques, such as distillation, solvent extraction, acid or base extraction, filtration and percolation are known to those skilled in the art.

Re-refined oils are obtained by processes similar to those used to obtain refined oils applied to refined oils which have been already used in service. Such re-refined oils are also known as reclaimed or reprocessed oils and often are additionally processed by techniques for approval of spent additive and oil breakdown products.

Hence, the base oil which may be used to make the present natural gas engine lubricating oil composition may be selected from any of the base oils in Groups I-V as specified in the American Petroleum Institute (API) Base Oil Interchangeability Guidelines (API Publication 1509). Such base oil groups are summarized in Table 1 below:

TABLE 1 Base Oil Properties Group^((a)) Saturates^((b)), wt. % Sulfur^((c)), wt. % Viscosity Index^((d)) Group I <90 and/or >0.03 80 to <120 Group II ≥90 ≤0.03 80 to <120 Group III ≥90 ≤0.03 ≥120 Group IV Polyalphaolefins (PAOs) Group V All other base stocks not included in Groups I, II, III or IV ^((a))Groups I-III are mineral oil base stocks. ^((b))Determined in accordance with ASTM D2007. ^((c))Determined in accordance with ASTM D2622, ASTM D3120, ASTM D4294 or ASTM D4927. ^((d))Determined in accordance with ASTM D2270.

Base oils suitable for use herein are any of the variety corresponding to API Group II, Group III, Group IV, and Group V oils and combinations thereof, preferably the Group III to Group V oils due to their exceptional volatility, stability, viscometric and cleanliness features.

The oil of lubricating viscosity for use in the lubricating oil compositions of this disclosure, also referred to as a base oil, is typically present in a major amount, e.g., an amount of greater than 50 wt. %, or greater than about 70 wt. %, or great than about 80%, based on the total weight of the lubricating oil composition. In one embodiment, the oil of lubricating viscosity can be present in the lubricating oil composition of this disclosure in an amount of less than about 90 wt. % or less than about 85 wt. %, based on the total weight of the lubricating oil composition. The base oil for use herein can be any presently known or later-discovered oil of lubricating viscosity used in formulating lubricating oil compositions for natural gas engines. Additionally, the base oils for use herein can optionally contain viscosity index improvers, e.g., polymeric alkylmethacrylates; olefinic copolymers, e.g., an ethylene-propylene copolymer or a styrene-butadiene copolymer; and the like and mixtures thereof. The topology of viscosity modifier could include, but is not limited to, linear, branched, hyperbranched, star, or comb topology.

As one skilled in the art would readily appreciate, the viscosity of the base oil is dependent upon the application. Accordingly, the viscosity of a base oil for use herein will ordinarily range from about 2 to about 2000 centistokes (cSt) at 1000 Centigrade (C.). Generally, individually the base oils used as engine oils will have a kinematic viscosity range at 100° C. of about 2 cSt to about 30 cSt, or about 3 cSt to about 16 cSt, or about 4 cSt to about 12 cSt and will be selected or blended depending on the desired end use and the additives in the finished oil to give the desired grade of engine oil, e.g., a lubricating oil composition having an SAE Viscosity Grade of 0W, 0W-8, 0W-12, 0W-16, 0W-20, 0W-30, 0W-40, 0W-50, 0W-60, 5W, 5W-20, 5W-30, 5W-40, 5W-50, 5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50, 15W, 15W-20, 15W-30, 15W-40, 30, 40 and the like.

The lubricating oil composition in accordance with the present disclosure further includes greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants. In one embodiment, the lubricating oil composition in accordance with the present disclosure will include greater than about 2.5 wt. % but less than about 3.5 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of the one or more phenolic antioxidants.

In one embodiment, the one or more phenolic antioxidants include, for example, sterically hindered phenolic antioxidants. Suitable one or more sterically hindered phenolic antioxidants include, for example, 2,6-di-tert-butylphenol (available under the trade designation IRGANOX™ L 140 from BASF), di-tert-butylated hydroxytoluene (“BHT”), methylene-4,4′-bis-(2,6-tert-butylphenol), 2,2′-methylene bis-(4,6-di-tert-butylphenol), 1,6-hexamethylene-bis-(3,5-di-tert-butyl-hydroxyhydrocinnamate) (available under the trade designation IRGANOX™ L109 from BASF), ((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)thio) acetic acid, C₁₀ to C₁₄ isoalkyl esters (available under the trade designation IRGANOX™ L118 from BASF), 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇ to C₉ alkyl esters (available under the trade designation IRGANOX™ L135 from BASF) tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionyloxymethyl) methane (available under the trade designation IRGANOX™ 1010 from BASF), thiodiethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate (available under the trade designation IRGANOX™ 1035 from BASF), octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (available under the trade designation IRGANOX™ 1076 from BASF) and 2,5-di-tert-butylhydroquinone.

The lubricating oil composition in accordance with the present disclosure further includes about 0.1 to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants. Suitable one or more aminic antioxidants include, for example, aromatic amines such as oil-soluble aromatic secondary amines, aromatic secondary polyamines and combinations thereof. Suitable aromatic secondary monoamines include, for example, diphenylamine, alkyl diphenylamines containing 1 or 2 alkyl substituents each having up to about 16 carbon atoms, phenyl-alpha-naphthylamine, phenyl-beta-napthylamine, alkyl- or aralkylsubstituted phenyl-alpha-naphthylamine containing at least one or two alkyl or aralkyl groups each having up to about 16 carbon atoms, alkyl- or aralkyl-substituted phenyl-beta-naphthylamine containing at least one or two alkyl or aralkyl groups each having up to about 16 carbon atoms, and the like.

In one embodiment, a suitable aromatic amine antioxidant is an alkylated diphenylamine of the general formula R¹—C₆H₄—NH—C₆H₄—R² wherein R¹ is a straight or branched alkyl group having about 6 to about 12 carbon atoms; and R² is a hydrogen atom or a straight or branched alkyl group having about 6 to about 12 carbon atoms. In one embodiment, R¹ and R² are the same. In one embodiment, a suitable aromatic amine antioxidant is a compound available commercially as Naugalube 438L (Lanxess), a material which is understood to be predominately a 4,4′-dinonyldiphenylamine (i.e., bis(4-nonylphenyl)(amine) wherein the nonyl groups are branched.

The lubricating oil composition in accordance with the present disclosure further includes one or more metal dithiophosphates. Suitable one or more metal dithiophosphates include, for example, zinc dialkyldithiophosphates, zinc diaryldithiophosphates and combinations thereof. In one embodiment, the one or more metal dithiophosphates include one or more zinc dialkyl dithiophosphate compounds derived from a primary alcohol. Suitable primary alcohols include those alcohols containing from 1 to 18 carbon atoms such as methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, dodecanol, octadecanol, propenol, butenol, and 2-ethylhexanol. In one embodiment, a zinc dialkyl dithiophosphate (ZnDTP) derived from a primary alcohol can be represented by a structure of formula (I):

Zn[S—P(═S)(OR¹)(OR²)]₂  (I)

wherein R¹ and R² may be the same or different alkyl radicals having from 1 to 18 carbon atoms or 2 to 12 carbon atoms or from 2 to 8 carbon atoms. The R¹ and R² groups of the zinc dialkyl dithiophosphate are derived from a primary alcohol as described above. In order to obtain oil solubility, the total number of carbon atoms (i.e., R¹+R²) will be at least 5.

In one embodiment, the one or more metal dithiophosphates include one or more zinc dialkyl dithiophosphate compounds derived from a secondary alcohol. Suitable secondary alcohols include those alcohols containing from 3 to 18 carbon atoms such as isopropyl alcohol, secondary butyl alcohol, isobutanol, 3-methylbutan-2-ol, 2-pentanol, 4-methyl-2-pentanol, 2-hexanol, 3-hexanol, and amyl alcohol. In one embodiment, a zinc dialkyl dithiophosphate derived from a secondary alcohol can be represented by a structure of formula (II):

Zn[S—P(═S)(OR¹)(OR²)]₂  (II)

wherein R¹ and R² may be the same or different alkyl radicals having from 3 to 18 carbon atoms or 3 to 12 carbon atoms or from 3 to 8 carbon atoms. The R¹ and R² groups of the zinc dialkyl dithiophosphate can be derived from the foregoing secondary alcohols. In order to obtain oil solubility, the total number of carbon atoms (i.e., R¹+R²) will be at least 5.

In one embodiment, the one or more metal dithiophosphates include a mixture of the foregoing zinc dialkyl dithiophosphate derived from a primary alcohol and zinc dialkyl dithiophosphate derived from a secondary alcohol. In general, the molar ratio of the primary alcohol to the secondary alcohol in the mixture of the one or more zinc dialkyl dithiophosphate compounds derived from a primary alcohol and one or more zinc dialkyl dithiophosphate compounds derived from a secondary alcohol can range from about 20:80 to about 80:20. In one embodiment, the molar ratio of the primary alcohol to the secondary alcohol in the mixture of the one or more zinc dialkyl dithiophosphate compounds derived from a primary alcohol and one or more zinc dialkyl dithiophosphate compounds derived from a secondary alcohol can range from about 30:70 to about 70:30. In one embodiment, the molar ratio of the primary alcohol to the secondary alcohol in the mixture of the one or more zinc dialkyl dithiophosphate compounds derived from a primary alcohol and one or more zinc dialkyl dithiophosphate compounds derived from a secondary alcohol can range from about 40:60 to about 60:40.

In general, the one or more metal dithiophosphates can be present in the lubricating oil composition of the present disclosure in an amount of about 1.5 wt. % or less, based on the total weight of the lubricating oil composition, e.g., an amount of about 0.08 wt. % to about 1.0 wt. %. In one embodiment, the one or more metal dithiophosphates can be present in the lubricating oil composition of the present disclosure in an amount of about 0.05 to about 0.8 wt. %, based on the total weight of the lubricating oil composition. In one embodiment, the one or more metal dithiophosphates can be present in the lubricating oil composition of the present disclosure in an amount of about 0.1 to about 0.7 wt. %, based on the total weight of the lubricating oil composition.

The lubricating oil composition in accordance with the present disclosure further includes one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.

The alkali metal or alkali earth metal phenate detergents can be prepared by methods known in the art. For example, the alkali metal or alkali earth metal phenate detergents can be prepared by neutralizing, overbasing and optionally sulfurizing an alkyl phenol in any order. Suitable alkyl groups include, for example, straight or branched chain C₁ to C₃₀ (e.g., C₄ to C₂₄) alkyl groups, or mixtures thereof. In one embodiment, suitable alkyl groups of the alkylphenol include those derived from one or more olefins comprising C₉ to Cis oligomers of monomers selected from propylene, butylene or mixtures thereof. Generally, the one or more olefins will contain a major mount of C₉ to C₁₈ oligomers of monomers selected from propylene, butylene or mixtures thereof. Examples of such olefins include propylene tetramer, butylene trimer and the like. As one skilled in the art will readily appreciate, other olefins may be present. For example, the other olefins that can be used in addition to the C₉ to C₁₈ oligomers include linear olefins, cyclic olefins, branched olefins other than propylene oligomers such as butylene or isobutylene oligomers, arylalkylenes and the like and mixtures thereof. Suitable linear olefins include 1-hexene, 1-nonene, 1-decene, 1-dodecene and the like and mixtures thereof. In another embodiment, suitable alkyl groups of alkylphenol include those derived from an isomerized normal-α-olefin such as an isomerized C₂₀ to C₂₄ normal-α-olefin. It should be noted that starting alkylphenols may contain more than one alkyl substituent that are each independently straight chain or branched chain.

The alkali metal or alkali earth metal phenate can be an overbased sulfurized calcium alkylphenate detergent which is substantially free of polyol promoter oxidation products. In general, the overbased sulfurized alkylphenate detergent which is substantially free of polyol promoter oxidation products is obtained by the process comprising (i) contacting an alkylphenol having at least one alkyl substituent from 6 to 36 carbon atoms with sulfur, in the presence of a promoter acid selected from the group of alkanoic acids having 1 through 3 carbon atoms, mixtures of the alkanoic acids, alkaline earth metal salts of the alkanoic acids and mixtures thereof, and at least a stoichiometric amount of a calcium base sufficient to neutralize the alkylphenol and the promoter at a temperature of from about 130° C. to about 250° C. under reactive conditions in the absence of a polyol promoter or an alkanol having 1 to 5 carbon atoms for a sufficient period of time to react essentially all of the sulfur thereby yielding a calcium sulfurized alkylphenate essentially free of elemental sulfur; and (ii) contacting the reaction product of step (i) with carbon dioxide and additional calcium base, if required; to provide the desired TBN, in the presence of an alkylene glycol having 2 to 6 carbon atoms under reactive conditions at a temperature of from about 150° C. to about 260° C., see, e.g., U.S. Pat. No. 5,529,705, the contents of which are incorporated by reference herein.

The process for preparing the sulfurized, calcium alkylphenate detergent can be conveniently conducted by contacting the desired alkylphenol with sulfur in the presence of a lower alkanoic acid and calcium base under reactive conditions. If desired, the alkylphenol can be contacted with sulfur in an inert compatible liquid hydrocarbon diluent. The reaction can be conducted under an inert gas, such as nitrogen, in theory the neutralization can be conducted as a separate step prior to sulfurization, but it is generally more convenient to conduct the sulfurization and the neutralization together in a single process step. Also, in place of the lower alkanoic acid, salts of the alkanoic acids or mixtures of the acids and salts could also be used. Where salts or mixtures of salts and acids are used, the salt is preferably an alkaline earth metal salt such as a calcium salt. In general, the acids are preferred and the process will be described below with respect to the use of lower alkanoic acid; however, it should be appreciated that the teachings are also applicable to the use of salts and mixtures of salts in place of all or a portion of the acids.

The combined neutralization and sulfurization reaction is typically conducted at temperatures in the range of about from about 115° C. to about 250° C. or from about 135° C. to about 230° C. depending on the particular alkanoic acid used. Where formic acid is used, a temperature in the range of about 150° C. to about 200° C. can be used. Where acetic acid or propionic acid are used, higher reaction temperatures may be advantageously employed, for example, at temperatures in the range of about 180° C. to about 250° C. or from about 200° C. to about 235° C.

If desired, mixtures of two or all three of the lower alkanoic acids also can be used. For example, mixtures containing about from about 5 to about 25 wt. % formic acid and about from about 75 to about 95 wt. % acetic acid can be used where low or medium overbased products are desired. Based on one mole of alkylphenol typically, from about 0.8 to about 3.5, or about 1.2 to about 2 moles of sulfur and about 0.025 to about 2, or about 0.1 to about 0.8 moles of lower alkanoic acid are used. Typically, about 0.3 to about 1 mole, or about 0.5 to about 0.8 mole of calcium base are employed per mole of alkylphenol.

In addition, an amount of calcium base sufficient to neutralize the lower alkanoic acid is also used. Thus, from about 0.31 to about 2 moles of calcium base are used per mole of alkylphenol including the base required to neutralize the lower alkanoic acid. If preferred, lower alkanoic acid to alkylphenol and calcium base to alkylphenol ratios are used, the total calcium base to alkylphenol ratio range will be about from about 0.55 to about 1.2 moles of calcium base per mole of alkylphenol. As one skilled in the art will readily appreciate, this additional calcium base will not be required where salts of alkanoic acids are used in place of the acids.

The reaction may be carried out in a compatible liquid diluent, such as a low viscosity mineral or synthetic oil. The reaction is conducted for a sufficient length of time to ensure complete reaction of the sulfur, e.g., where high TBN products are desired because the synthesis of such products generally requires using carbon dioxide together with a polyol promoter. Accordingly, any unreacted sulfur remaining in the reaction mixture will catalyze the formation of deleterious oxidation products of the polyol promoter during the overbasing step.

Where the neutralization is conducted as a separate step, both the neutralization and the subsequent sulfurization are conducted under the same conditions as set forth above. In either case, it is desirable to remove water generated by the neutralization of the alkylphenol. This is conventional and generally is accomplished by continuous distillation during the neutralization. Conveniently, a high molecular weight alkanol having 8 to 16 carbon atoms may be added to the neutralization-sulfurization step and/or the overbasing step as a solvent and also to assist in the removal of water by forming a water-azeotrope which may then be distilled off.

Optionally, specialized sulfurization catalysts such as those described in U.S. Pat. No. 4,744,921, the contents of which is incorporated by reference herein, can be employed in the neutralization-sulfurization reaction together with the lower alkanoic acid. However, any benefit afforded by the sulfurization catalyst, for example, reduced reaction time, is offset by the increase in costs incurred by the catalyst and/or the presence of undesired residues in the case of halide catalysts or alkali metal sulfides; especially, as excellent reaction rates can be obtained by merely using acetic and/or propionic acid and increasing reaction temperatures.

The alkali metal or alkali earth metal phenate detergents may be a phenate detergent with reduced unsulfurized tetrapropenylphenol content (TPP). Methods to reduce TPP are well documented in the literature, which includes, for example, extraction, distillation, steam-stripping, Mannich post-treatment, and other manufacture and post-treatment processes. In one embodiment, the alkali metal or alkali earth metal phenate detergent may be a distilled phenate detergent, where the distillation occurs after the neutralization but before sulfurization step. In another embodiment, the alkali metal or alkali earth metal phenate detergent may be a distilled phenate detergent, where the distillation occurs after sulfurization and neutralization, and optionally the overbasing step. In another embodiment, the alkali metal or alkali earth metal phenate detergent may be a solvent-extracted phenate detergent. In one embodiment, the alkali metal or alkali earth metal phenate detergent has a reduced TPP content of 3.4 wt. % or less.

In one aspect of the present disclosure, the alkali metal or alkali earth metal phenate detergent is a calcium non-sulfurized phenate detergent having a TBN of about 150 to about 250 on an oil free basis, wherein the calcium phenate detergent provides at least about 0.10 wt. % of calcium to the natural gas engine lubricating oil composition.

In one aspect of the present disclosure, the alkali metal or alkali earth metal phenate detergent is a calcium sulfurized phenate detergent having a TBN of about 150 to about 250 on an oil free basis, wherein the calcium phenate detergent provides at least about 0.10 wt. % of calcium to the natural gas engine lubricating oil composition.

In general, the alkali metal or alkali earth metal phenate detergents are present in an amount that provides at least about 0.10 wt. of metal to the natural gas engine lubricating oil composition. In one embodiment, the alkali metal or alkali earth metal phenate detergents are present in an amount that provides about 1000 ppm to about 2500 ppm of metal to the natural gas engine lubricating oil composition. In one embodiment, the alkali metal or alkali earth metal phenate detergents are present in an amount that provides about 1200 ppm to about 2000 ppm of metal to the natural gas engine lubricating oil composition.

In one embodiment, the alkali metal or alkali earth metal phenate detergents are present in the lubricating oil composition in an amount of about 1 wt. % to about 5 wt. %, based on the total weight of the natural gas engine lubricating oil composition. In one embodiment, the alkali metal or alkali earth metal phenate detergents are present in the lubricating oil composition in an amount of about 1.5 wt. % to about 4.5 wt. %, based on the total weight of the natural gas engine lubricating oil composition.

The lubricating oil compositions of the present disclosure may also contain other conventional additives that can impart or improve any desirable property of the lubricating oil composition in which these additives are dispersed or dissolved. Any additive known to a person of ordinary skill in the art may be used in the lubricating oil compositions disclosed herein. Some suitable additives have been described in Mortier et al., “Chemistry and Technology of Lubricants”, 2nd Edition, London, Springer, (1996); and Leslie R. Rudnick, “Lubricant Additives: Chemistry and Applications”, New York, Marcel Dekker (2003), both of which are incorporated herein by reference. For example, the natural gas engine lubricating oil compositions can be blended with antioxidants, detergents, rust inhibitors, dehazing agents, demulsifying agents, metal deactivating agents, friction modifiers, pour point depressants, antifoaming agents, co-solvents, corrosion-inhibitors, ashless dispersants, multifunctional agents, dyes, extreme pressure agents and the like and mixtures thereof. A variety of the additives are known and commercially available. These additives, or their analogous compounds, can be employed for the preparation of the lubricating oil compositions of the disclosure by the usual blending procedures.

Suitable alkali metal or alkaline earth metal detergents include, for example, alkali metal or alkaline earth metal sulfonates, alkali metal or alkaline earth metal salicylates, alkali metal or alkaline earth metal phosphonates, alkali metal or alkaline earth metal thiophosphonates and combinations thereof.

In one embodiment, the alkali metal or alkaline earth metal detergents can be overbased alkali metal or alkaline earth metal detergents. In one embodiment, the alkali metal or alkaline earth metal detergents have a TBN (oil free basis) of 0 to about 60. In another embodiment, the alkali metal or alkaline earth metal detergents have a TBN (oil free basis) of greater than 60 to about 200. In another embodiment, the alkali metal or alkaline earth metal detergents have a TBN (oil free basis) of greater than about 200 to about 800.

A dispersant is an additive whose primary function is to hold solid and liquid contaminations in suspension, thereby passivating them and reducing engine deposits at the same time as reducing sludge depositions. For example, a dispersant maintains in suspension oil-insoluble substances that result from oxidation during use of the lubricant, thus preventing sludge flocculation and precipitation or deposition on metal parts of the engine.

Dispersants are usually “ashless”, being non-metallic organic materials that form substantially no ash on combustion, in contrast to metal-containing, and hence ash-forming materials. They comprise a long hydrocarbon chain with a polar head, the polarity being derived from inclusion of at least one nitrogen, oxygen or phosphorus atom. The hydrocarbon is an oleophilic group that confers oil-solubility, having, for example, 40 to 500 carbon atoms. Thus, ashless dispersants may comprise an oil-soluble polymeric backbone.

One class of olefin polymers is constituted by polybutylenes, specifically polyisobutylenes (PIB) or poly-n-butylenes, such as may be prepared by polymerization of a C4 refinery stream. Dispersants include, for example, derivatives of long chain hydrocarbonsubstituted carboxylic acids, examples being derivatives of high molecular weight hydrocarbyl-substituted succinic acid. A noteworthy group of dispersants is constituted by hydrocarbon-substituted succinimides, made, for example, by reacting the above acids (or derivatives) with a nitrogen-containing compound, advantageously a polyalkylene polyamine, such as a polyethylene polyamine. Typical commercially available polyisobutylene-based succinimide dispersants contain polyisobutylene polymers having a number average molecular weight ranging from 900 to 2500, functionalized by maleic anhydride, and derivatized with polyamines having a molecular weight of from 100 to 350.

Other suitable dispersants include succinic esters and ester-amides, Mannich bases, polyisobutylene succinic acid (PIBSA), and other related components.

Succinic esters are formed by the condensation reaction between hydrocarbon-substituted succinic anhydrides and alcohols or polyols. For example, the condensation product of a hydrocarbon-substituted succinic anhydride and pentaerythritol is a useful dispersant.

Succinic ester-amides are formed by condensation reaction between hydrocarbon-substituted succinic anhydrides and alkanol amines. For example, suitable alkanol amines include ethoxylated polyalkylpolyamines, propoxylated polyalkylpolyamines and polyalkenylpolyamines such as polyethylene polyamines. One example is propoxylated hexamethylenediamine.

Mannich bases are made from the reaction of an alkylphenols, formaldehyde, and a polyalkylene polyamines. Molecular weights of the alkylphenol may range from 800 to 2500.

Nitrogen-containing dispersants may be post-treated by conventional methods to improve their properties by reaction with any of a variety of agents. Among these are boron compounds (e.g., boric acid) and cyclic carbonates (e.g., ethylene carbonate).

A friction modifier is any material or materials that can alter the coefficient of friction of a surface lubricated by any lubricant or fluid containing such material(s). Suitable friction modifiers include, for example alkoxylated fatty amines, borated fatty epoxides, fatty phosphites, fatty epoxides, fatty amines, borated alkoxylated fatty amines, metal salts of fatty acids, fatty acid amides, glycerol esters, borated glycerol esters and fatty imidazolines. As used herein, the term “fatty” means a hydrocarbon chain having 10 to 22 carbon atoms, typically a straight hydrocarbon chain.

Other known friction modifiers comprise oil-soluble organo-molybdenum compounds. Such organo-molybdenum friction modifiers also provide antioxidant and anti-wear credits to a lubricating oil composition. Suitable oil-soluble organomolybdenum compounds have a molybdenum-sulfur core. As examples, there may be mentioned dithiocarbamates, dithiophosphates, dithiophosphinates, xanthates, thioxanthates, sulfides, and mixtures thereof. The molybdenum compound may be dinuclear or trinuclear.

Corrosion inhibitors protect lubricated metal surfaces against chemical attack by water or other contaminants. Suitable corrosion inhibitors include, for example, polyoxyalkylene polyols and esters thereof, polyoxyalkylene phenols, thiadiazoles and anionic alkyl sulfonic acids.

Viscosity modifiers provide lubricants with high and low temperature operability. These additives increase the viscosity of the oil composition at elevated temperatures which increases film thickness, while having limited effect on viscosity at low temperatures.

Suitable viscosity improvers include, for example, high molecular weight hydrocarbons, polyesters and viscosity index improver dispersants that function as both a viscosity index improver and a dispersant. Typical molecular weights of these polymers are in a range of about 1000 to about 1,000,000 (e.g., about 2000 to about 500,000 or about 25,000 to about 100,000).

Examples of suitable viscosity improvers are polymers and copolymers of methacrylate, butadiene, olefins, or alkylated styrenes. Polyisobutylene is a commonly used viscosity modifier. Another suitable viscosity modifier is polymethacrylate (copolymers of various chain length alkyl methacrylates, for example), some formulations of which also serve as pour point depressants. Other suitable viscosity modifiers include copolymers of ethylene and propylene, hydrogenated block copolymers of styrene and isoprene, and polyacrylates (copolymers of various chain length acrylates, for example). Specific examples include styrene-isoprene or styrenebutadiene based polymers of 50,000 to 200,000 molecular weight.

Pour point depressants lower the minimum temperature at which a fluid will flow or can be poured. Suitable pour point depressants include, for example, C₈ to C₁₈ dialkyl fumarate/vinyl acetate copolymers, polyalkylmethacrylates and the like.

Foam inhibitors retard the formation of stable foams. Suitable foam inhibitors include, for example, polysiloxanes, polyacrylates, and the like.

In the preparation of lubricating oil formulations, it is common practice to introduce the additives in the form of about 10 to about 80 wt. % active ingredient concentrates in hydrocarbon oil, e.g. mineral lubricating oil, or other suitable solvent.

Usually these concentrates may be diluted with about 3 to about 100, e.g., about 5 to about 40, parts by weight of lubricating oil per part by weight of the additive package in forming finished lubricants, e.g. crankcase motor oils. The purpose of concentrates, of course, is to make the handling of the various materials less difficult and awkward as well as to facilitate solution or dispersion in the final blend.

Each of the foregoing additives, when used, is used at a functionally effective amount to impart the desired properties to the lubricant. Thus, for example, if an additive is a friction modifier, a functionally effective amount of this friction modifier would be an amount sufficient to impart the desired friction modifying characteristics to the lubricant.

In general, the concentration of each of the additives in the lubricating oil composition, when used, may range from about 0.001 wt. % to about 20 wt. %, or from about 0.005 wt. % to about 15 wt. %, or from about 0.01 wt. % to about 10 wt. %, or from about 0.1 wt. % to about 5 wt. %, or from about 0.1 wt. % to about 2.5 wt. %, based on the total weight of the lubricating oil composition. Further, the total amount of the additives in the lubricating oil composition may range from about 0.001 wt. % to about 20 wt. %, or from about 0.01 wt. % to about 10 wt. %, or from about 0.1 wt. % to about 5 wt. %, based on the total weight of the lubricating oil composition.

The following examples are presented to exemplify embodiments of the disclosure but are not intended to limit the disclosure to the specific embodiments set forth. Specific details described in each example should not be construed as necessary features of the disclosure. The following examples are intended for illustrative purposes only and do not limit in any way the scope of the present disclosure. All numerical values are approximate. When numerical ranges are given, it should be understood that embodiments outside the stated ranges may still fall within the scope of the disclosure.

To determine the effect of additives on deposit control in an engine, lubricating oil compositions were prepared having the formulations set forth in the following examples. The compositions were prepared by mixing the base oil(s) with additive packages according to conventional preparation methods. Deposit performance of the lubricant oil compositions was measured using the TEOST MHT-4 with depoisitor rod temperature at 285° C.

EXAMPLES Baseline Formulation 1

A baseline formulation was prepared by adding a mixture of succinimide dispersants, 200 TBN (oil-free basis) Ca phenate providing 0.15 wt % Ca (to the lubricant composition), ZnDTP providing 0.26 wt % P, MOB Ca carboxylate providing 0.12 wt % Ca, LOB Ca sulfonate providing 0.28 wt % of Ca, aminic antioxidant, demulsifier, foam inhibitor and Group II base oil.

Example 1

A natural gas engine lubricating oil composition was prepared by adding 3.00 wt % hindered phenolic antioxidant to the baseline formulation.

Example 2

A natural gas engine lubricating oil composition was prepared by adding 3.30 wt % hindered phenolic antioxidant to the baseline formulation.

Comparative Example 1

A natural gas engine lubricating oil composition was prepared by adding 1.00 wt % hindered phenolic antioxidant to the baseline formulation.

Comparative Example 2

A natural gas engine lubricating oil composition was prepared by adding 1.50 wt % hindered phenolic antioxidant to the baseline formulation.

Comparative Example 3

A natural gas engine lubricating oil composition was prepared by adding 2.00 wt % hindered phenolic antioxidant to the baseline formulation.

TEOST MHT4 Test

TEOST MHT4 is a bench test requirement used in gasoline engine oil categories such as ILSAC GF-5 and API SN Resource Conserving for performance category GF-5. ASTM D7097 is designed to predict the deposit-forming tendencies of engine oil in the piston ring belt and upper piston crown area. Correlation has been shown between the TEOST MHT procedure and the TU3MH Peugeot engine test in deposit formation. This test determines the mass of deposit formed on a specially constructed test rod exposed to repetitive passage of 8.5 g of engine oil over the rod in a thin film under oxidative and catalytic conditions at 285 degC. Deposit-forming tendencies of an engine oil under oxidative conditions are determined by circulating an oil-catalyst mixture comprising a small sample (8.4 g) of the oil and a very small (0.1 g) amount of an organo-metallic catalyst. This mixture is circulated for 24 hours in the TEOST MHT instrument over a special wire-wound depositor rod heated by electrical current to a controlled temperature of 285 degC at the hottest location on the rod. The rod is weighed before and after the test. Deposit that fell off the depositor rod into the oil was filtered and weighed. Total deposit is sum of the weight of deposits on depositor rod and on the filter. Here, a deposit weight of less than 12 mg is considered to pass the pass/fail criteria.

TABLE 2 Compar- Compar- Compar- Inventive Inventive ative ative ative Example 1 Example 2 Example 1 Example 2 Example 3 Total 8 6 19 15 13 Deposit mg

The TEOST data show Inventive Example 1 and 2 have total deposit less than 12 mg, while Comparative Example 1-3 have total deposit greater than 12 mg.

It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. For example, the functions described above and implemented as the best mode for operating the present disclosure are for illustration purposes only. Other arrangements and methods may be implemented by those skilled in the art without departing from the scope and spirit of this disclosure. Moreover, those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

What is claimed is:
 1. A natural gas engine lubricating oil composition comprising: (a) a major amount of an oil of lubricating viscosity, (b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants, (c) about 0.1 wt. % to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants, (d) one or more metal dithiophosphates, (e) one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition,
 2. The natural gas engine lubricating oil composition of claim 1, having a TBN of about 5 to about 8 as determined by ASTM D
 2896. 3. The natural gas engine lubricating oil composition of claim 1, having a TBN of about 6 to about 7 as determined by ASTM D
 2896. 4. The natural gas engine lubricating oil composition of claim 1, having a sulfated ash content of about 0.65 to about 0.8 wt. % as determined by ASTM D
 874. 5. The natural gas engine lubricating oil composition of claim 1, wherein the major amount of the oil of lubricating viscosity is greater than 50 wt. %, based on the total weight of the lubricating oil composition.
 6. The natural gas engine lubricating oil composition of claim 1, wherein the one or more phenolic antioxidants are one or more sterically hindered phenolic antioxidants.
 7. The natural gas engine lubricating oil composition of claim 1, wherein the one or more aminic antioxidants are one or more diphenylamine antioxidants.
 8. The natural gas engine lubricating oil composition of claim 1, wherein the one or more metal dithiophosphates are one or more zinc dithiophosphates.
 9. The natural gas engine lubricating oil composition of claim 1, wherein the one or more metal dithiophosphates are one or more zinc dialkyl dithiophosphate compounds derived from a primary alcohol.
 10. The natural gas engine lubricating oil composition of claim 1, comprising about 0.01 wt. % to about 0.3 wt. % of phosphorus derived from the one or more metal dithiophosphates, based on the total weight of the lubricating oil composition.
 11. The natural gas engine lubricating oil composition of claim 1, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.12 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.
 12. The natural gas engine lubricating oil composition of claim 1, wherein the one or more alkali metal or alkali earth metal phenate detergents are one or more calcium phenate detergents.
 13. The natural gas engine lubricating oil composition of claim 1, wherein the one or more alkali metal or alkali earth metal phenate detergents are one or more sulfurized calcium phenate detergents.
 14. The natural gas engine lubricating oil composition of claim 1, further comprising at least one additive selected from the group consisting of antioxidants, rust inhibitors, dehazing agents, metal detergents, demulsifying agents, metal deactivating agents, friction modifiers, pour point depressants, antifoaming agents, co-solvents, corrosion-inhibitors, ashless dispersants, multifunctional agents, dyes, extreme pressure agents and mixtures thereof.
 15. A method for preventing or inhibiting deposit formation in a natural gas engine containing one or more steel pistons comprising the step of operating the natural gas engine with a natural gas engine lubricating oil composition comprising: (a) a major amount of an oil of lubricating viscosity, (b) greater than about 2 wt. % but less than about 4 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more phenolic antioxidants, (c) about 0.1 wt. % to about 1 wt. %, based on the total weight of the natural gas engine lubricating oil composition, of one or more aminic antioxidants, (d) one or more metal dithiophosphates, and (e) one or more alkali metal or alkali earth metal phenate detergents having a total base number (TBN) of about 150 to about 250 on an oil free basis, wherein the one or more alkali metal or alkali earth metal phenate detergents provide at least about 0.10 wt. % of alkali metal or alkali earth metal to the natural gas engine lubricating oil composition.
 16. The method of claim 15, wherein the natural gas engine lubricating oil composition has a TBN of about 5 to about 8 and a sulfated ash content of about 0.65 to about 0.8 wt. % as determined by ASTM D
 874. 17. The method of claim 15, wherein the one or more phenolic antioxidants are one or more sterically hindered phenolic antioxidants, the one or more aminic antioxidants are one or more diphenylamine antioxidants, and the one or more metal dithiophosphates are one or more zinc dithiophosphates.
 18. The method of claim 15, wherein the natural gas engine lubricating oil composition comprises about 0.01 wt. % to about 0.3 wt. % of phosphorus derived from the one or more metal dithiophosphates, based on the total weight of the lubricating oil composition.
 19. The method of claim 15, wherein the one or more alkali metal or alkali earth metal phenate detergents are one or more sulfurized calcium phenate detergents.
 20. The method of claim 15, wherein the lubricating oil composition further comprises at least one additive selected from the group consisting of antioxidants, rust inhibitors, dehazing agents, demulsifying agents, metal deactivating agents, friction modifiers, pour point depressants, antifoaming agents, co-solvents, corrosion-inhibitors, ashless dispersants, multifunctional agents, dyes, extreme pressure agents and mixtures thereof.
 21. The method of claim 15, wherein the deposit formed in the natural gas engine is less than 12 mg. 